docker: crosscompile

Dockerfile

@@ -1,10 +1,11 @@
-FROM golang AS build
+FROM --platform=$BUILDPLATFORM golang AS build
 
 ARG GIT_DESC=undefined
 
 WORKDIR /go/src/github.com/Snawoot/dtlspipe
 COPY . .
-RUN CGO_ENABLED=0 go build -a -tags netgo -ldflags '-s -w -extldflags "-static" -X main.version='"$GIT_DESC" ./cmd/dtlspipe
+ARG TARGETOS TARGETARCH
+RUN GOOS=$TARGETOS GOARCH=$TARGETARCH CGO_ENABLED=0 go build -a -tags netgo -ldflags '-s -w -extldflags "-static" -X main.version='"$GIT_DESC" ./cmd/dtlspipe
 
 FROM scratch
 COPY --from=build /go/src/github.com/Snawoot/dtlspipe/dtlspipe /